In recent years, high-strength steel sheets have been widely used in many automotive parts to realize weight reduction of automotive bodies to respond to environmental issues. To manufacture automotive parts, press forming processes are often used because press forming processes are excellent in view of manufacturing costs. However, since such high-strength steel sheets have a lower level of ductility than low-strength steel sheets, during a deep drawing process in which a blank holder force is applied while a tip end portion of a blank is gripped by a blank holder, a large strain tends to occur in the vicinity of the tip end portion of the blank, which, may easily lead to a fracture of the sheet.
Therefore, in such a case, a crash forming process is preferably used in many occasions, which is a kind of press forming process that primarily uses a bending deformation process without using the blank holder. However, during such crash forming processes, because the tension applied to a blank is small, excess metal caused by a shape of the manufactured component part may easily become a direct cause of wrinkles. It is therefore difficult to manufacture a press-formed product having a desired shape.
Patent Literature 1 describes a method for manufacturing an L-shaped product by performing a pressing process. According to this method, it is indicated that, by performing the pressing process while using a pad for preventing wrinkles, it is possible to avoid the occurrence of wrinkles in the top portion of the L-shape product and to avoid the occurrence of a fracture during a stretch flange forming process.
Patent Literature 2 discloses a method for manufacturing a component part that has an arc-shaped portion in a corner portion thereof, without generating wrinkles in a side wall portion thereof. This method includes: a step of manufacturing an intermediate formed product having no arc-shaped portion formed therein; and a step of completing the corner portion by performing a drawing process to form the arc-shaped portion in the intermediate formed product manufactured at the preceding step. It is indicated that it is possible to avoid the occurrence of wrinkles by making one or more incisions in such a section that does not reach the arc-shaped portion, the incisions each starting from the edge side of a flange portion.
Patent Literature 3 discloses a tool of press forming which applies a bending deformation process to make a hat-shaped cross section from a steel blank sheet and immediately after the bending deformation process is completed, to apply a compressive stress to a side wall portion of the steel blank sheet.